This invention relates to a method of preparing stable dilute solutions of aliphatic carboxylic peracids, inter alia monoperacetic acid.
It is known that solutions of monoperacetic acid prepared from hydrogen peroxide and acetic acid reach equilibrium in accordance with an equation which can be written as follows: ##STR1##
The kinetics of this system are relatively slow; the reaction rates are accelerated by strong acids such as sulphuric acid H.sub.2 SO.sub.4 or phosphoric acid H.sub.3 PO.sub.4 which, in the present case, act as catalysts. The presence of these strong acids enables the system to move more rapidly towards equilibrium.
It is known, however, that once equilibrium has been reached, the system slowly loses the active oxygen which it contains. This loss results in a slow decrease in the content of peracid and hydrogen peroxide. Stabilizers are added to counteract this phenomenon.
All these phenomena have been demonstrated inter alia by F. K. Greenspan (J. Amer. Chem. Soc., 1946, 68, 907). The author also observed that concentrated solutions are relatively stable, so that peracetic acid can be sold at high concentrations in stabilized solutions. At present, monoperacetic acid is conventionally sold at concentrations of 35-45%.
However, dilute solutions of aliphatic percarboxylic acids which are stable for a prolonged period are required in other applications of monoperacetic acid, e.g. for disinfection and sterilization in medical and food applications.
Various methods have been proposed for obtaining dilute peracetic acid solutions having a concentration between 0.5 and 20%. FMC Corporation's French Pat. No. 1 352 479 relates to a continuous process, by reacting a lower aliphatic acid with hydrogen peroxide in the proportion of 0.3 to 5 mols hydrogen peroxide per mol of acid, in the presence of 5 to 20% of a strong dehydrating acid. French Pat. No. 1 452 484 by the same company relates to a method of producing dilute aqueous solutions of peracetic acid having a concentration of 0.5 to 7% by weight of peracetic acid and a pH of 5.4-7 and free from diacetyl peroxide.
The object of these methods of preparing peracetic acid is to separate the peracetic acid after it has been formed in the reaction mixture. This is either because of the secondary reactions caused by sulphuric acid, H.sub.2 O.sub.2 and acetic acid, e.g. in epoxidation reactions, or because of the safety problems resulting from the presence of diacetyl peroxide in bleaching operations.
In other applications of peracetic acid, e.g. for disinfection or sterilization, it is unnecessary to separate the product acid from the reaction mixture; solutions of peracetic acid obtained before separation, by the methods described in the preceding patent specifications, can be used for sterilization and disinfection. The same applies to the peracetic acid solutions obtained by Greenspan from 1946 onwards.
More recently, solutions containing 0.5 to 20% of peracid having 2-3 carbon atoms and/or the corresponding aliphatic monocarboxylic acids, 25-40% hydrogen peroxide and 0.05 to 5% by weight of anionic wetting agent in the form of alkylbenzene sulphonate, alkyl sulphate and/or alkane sulphonate, the remainder being water, having been disclosed in French patent application No. 2 321 301. Concentrated solutions containing 0.25 to 10% phosphonic acid or the corresponding water-soluble acid salts are described in French patent application No. 2 321 302; both of these are in the name of Henkel & Co. GmbH.
In any case, if it is desired to manufacture a weak solution of peracetic acid under industrial conditions and even to sell this, it is thought necessary, or at least desirable, that the product solutions must meet the following two basic criteria:
(1) Industrial manufacture of the solutions must be rapid enough to avoid uneconomic storage of the reaction mixtures so as to obtain the desired concentration of peracetic acid. In this connection, the reaction mixture must attain at least 90% of the maximum equilibrium concentration within 48 hours.
(2) The stability of the peracetic acid solution at ambient temperature must be such that the concentration of peracetic acid after storage for 12 months is not below 90% the maximum concentration obtained by the reaction mixture; the maximum concentration may incidentally be different from the rated concentration of the mixture.
In other words, a solution of peracetic acid sold at the rated concentration of 5%, for example, must have a concentration of 5% when sold, 5.55% at maximum concentration and 5% after storage for 12 months at ambient temperature. A process has now been found which meets industrial requirements by solving the inherent difficulties of high-speed production and prolonged storage of dilute solutions of aliphatic carboxylic acids having a concentration of peracid, which does not vary greatly over a prolonged period.